The goal of this project is to understand the role of Gal/GalNAc lectins in the development and function of the immune system. As cells migrate from the thymic cortex to the medulla, there is alpha2-3 sialylation of Galbeta1-3GalNAc moities, causing a loss of binding by the plant lectin peanut agglutinin (PNA). This conversion from PNA+ to PNA- positive phenotype appears to result from the regulated expression of a Gal beta1,3 GalNAc alpha2,3- sialyltransferase (ST3Gal I). Many blood cells are PNA- due to this sialylation, and thus become PNA+ when treated with a sialidase. Exceptions (in addition to immature thymocytes) that are PNA+ include germinal center B cells and plasmacytes. The significance of this regulated sialylation is underscored by its phylogenetic conservation between lizards, chickens, mice, rats, and humans. One hypothesis is that lymphoid migration or the uptake of apoptotic cells within primary and secondary organs is dependent, in part, -on Galbeta-3Ga1NAc binding by a specific receptor present on macrophages within the thymus, lymph node, and bone marrow. Studies on Gal/GalNAc receptors show that there is a single polypeptide lectin that is expressed on some, but not all macrophages. This receptor has been termed termed MMGL for mouse macrophage galactose/N-acetylgalactosamine-specific C-type lectin. In preliminary data provided we have shown that there is a distinct, anatomically localized population of cells in the thymus and spleen that express a Gal/GalNAc lectin or lectins. Using monoclonal antibodies we have also shown that a similar population expresses MMGL. Using a combination of histology, biochemistry, flow cytometry and genetics we will determine the identity of the Gal/GalNAc lectins in primary and second lymphoid organs. We will produce mice deficient in the gene encoding MMGL and determine how this deficiency affects the development, maturation, and function of T cells, B cells and macrophages. Similarly, we will study the immune development that takes place in mice that are are deficient in the gene encoding ST3Gal I (Provided by Project I). We predict that the thymocytes from these mice will remain PNA+ throughout development. Since little is known about the role of Gal/GalNAc lectin interactions in lymphoid development these studies are likely to reveal new types of cellular interactions critical to the organization and function of the immune system.